This invention relates to heat exchanger apparatus and methods for transfer of heat to fluids and, more particularly, to a fluid heater apparatus and method of heating heat transfer fluids. The invention is described below with particular reference to heat transfer fluid applications.
Heat transfer fluids are heated to desired working temperatures for transfer of process heat in a wide range of industrial applications such as indirect heating of process liquids and polymers, heating and cooling in batch processing, energy generation and recovery processes, drying and heating bulk materials and gas processing. Typical industrial applications include textile, chemical, energy and hydrocarbon processing. These heat transfer fluids are typically useful in temperature ranges between about 350.degree. to about 800.degree. F. There are two types of fluids, the so called hot oils derived from petroleum and synthetic fluids such as aromatic fluids sold under the designation DOWTHERM by The Dow Chemical Company.
When exposed to high temperatures, such heat transfer fluids undergo physical and chemical changes that shorten the useful fluid life and result in a loss of useable fluid. Such thermal degradation of the fluids may also reduce the heat transfer efficiency and system safety while at the same time increasing operating costs and downtime.
The heat transfer fluids are typically heated as they flow through heater or heating tubes of a heat exchanger apparatus. In such cases, the liquid flow within the heater tube is characterized by a temperature profile that includes a relatively higher film temperature at radially outward regions in the heater tube and a relatively lower bulk fluid temperature at radially inward regions adjacent the central core region of the tube. The film temperature may be 50.degree. to 75.degree. F. higher than the bulk fluid temperature.
The relatively higher film temperature tends to limit the designed upper heating temperature and heat flux transfer in such heating apparatus and applications. More particularly, the possibility of heat flux variations and associated higher operating temperatures require a buffer or safety range between the designed operating temperature and the upper thermal limit of the heat transfer fluid. For similar reasons, it is desirable that the heater tube not be directly impinged by a heating flame since this may result in local fluid temperature increases of several hundred degrees Fahrenheit and sometimes leave a deposit on the interior wall of the heater tube. Such deposits decrease the heat transfer and lower the heat transfer efficiency.
The foregoing difficulties in maintaining a design operating temperature substantially at or near the upper thermal limit of the heat transfer fluid are further complicated in a heating application requiring a turn down capability. In such cases, the required safety margin for heat transfer at high operating temperature conditions may result in an extremely inefficient or slow response at low operating temperature conditions.
It is also desirable that the heat exchanger be compact since it will often comprise an ancillary device or a component of a more comprehensive apparatus. Thus, it is not desirable to merely extend the Spatial arrangement of the heating flame and the heater tube with variable flame operation along the extent of the tube to accommodate turn down. Similarly, the prior art use of excess air to suppress flame temperatures, reduced BTU input rates, shorter flame lengths and/or separate combustion chambers are not satisfactory techniques for avoiding thermal degradation of the heat transfer fluid since such techniques also result in increased heat exchanger size and inefficient operation and decreased response times also result.
Heating apparatus including a central heat flame and a surrounding heater tube array is disclosed in U.S. Pat. Nos. 4,793,800, 4,723,513, 4,473,034, 4,444,155 and 4,338,888. U.S. Pat. No. 4,679,528 discloses a heating boiler having a central heating flame and a surrounding coil heater tube. The products of combustion pass radially through the heater tube coil, and are discharged through an axially extending vent.